1. Field of the Invention
The invention generally relates to microlithographic projection exposure apparatuses as are used in the manufacture of integrated circuits and other microstructured devices. More particularly, the invention relates to a method for improving an optical imaging property, for example spherical aberration or the focal length, of a projection objective of such an apparatus. The invention further relates to a microlithographic projection exposure apparatus as such.
2. Description of Related Art
It is known to change the spatial position of individual optical components, for example by using manipulators, in a projection objective of a microlithographic projection exposure apparatus in order to improve the imaging properties of the objective. The positional change of the relevant optical components is in this case carried out on the projection objective once it has finally been installed, and in general before it has yet been put into operation for the first time. This type of fine adjustment may nevertheless be carried out at a later time, for example in order to compensate for deteriorations of the imaging properties due to ageing. A procedure often adopted in these methods is to record one or more imaging properties of the projection objective by using a sensor arranged in its image plane. The way in which positional changes of individual optical components affect the imaging properties is then observed. The optical imaging properties of the projection objective can thus be optimised by adjusting the optical components.
From EP 0 023 231 B1 a microlithographic projection exposure apparatus is known that has, in order to hold a support for a semiconductor wafer to be exposed, an open-topped container whose upper edge is higher than the lower delimiting surface of the projection objective. The container is provided with feed and discharge lines for an immersion liquid, which is circulated in a liquid circuit. When the projection exposure apparatus is in operation, the immersion liquid fills the interspace which is left between the semiconductor wafer to be exposed and a boundary surface of the projection exposure objective, which faces it. The resolving power of the projection objective is increased because the refractive index of the immersion liquid is higher than that of air.
The known projection exposure apparatus furthermore has a device for regulating the temperature of the immersion liquid, which is arranged in the liquid circuit. The temperature of the semiconductor wafer to be exposed can thereby be kept constant, so as to avoid imaging errors due to thermally induced movements of the semiconductor wafer.
The use of immersion liquids in microlithographic projection exposure apparatus is also known from JP 10-303 114 A. This addresses the problem that undesirable temperature fluctuations of the immersion liquid can also cause a deterioration of the imaging properties of the projection objective. The reason for this involves the dependency of the refractive index of the immersion liquid on the temperature. In order to resolve this problem, various measures are proposed by which the temperature of the immersion liquid can be kept constant within narrow limits during operation of the projection exposure apparatus.